int N = 12; // Number of time intervals
int l = 4;  // Number of channels
int K = 11; // Number of off_Takes or irrigations

range TimeIntervals = 1..N;
range Channels = 1..l;
range Irrigations = 1..K;

float gamma[Channels] = [0.1, 0.1, 0.1, 0.1]; // Gate operation delay factor
int Ii[Channels] = [0, 1, 2, 2]; // Set of the sets of the channels downstream every channel 

Hi,

The following model works:

int N = 12; // Number of time intervals
int l = 4;  // Number of channels
int K = 11; // Number of off_Takes or irrigations

range Operations = 1..1;
range TimeIntervals = 1..N;
range Channels = 1..l;
range Irrigations = 1..K;

float gamma[Channels] = [0.1, 0.1, 0.1, 0.1]; // Gate operation delay factor
int Ii[Channels] = [0, 1, 2, 2]; // Set of the sets of the channels downstream every channel 
int tau[Channels] = [120, 60, 60, 60]; // Delay times for every channel
 
int R0[Channels] = [0, 0, 0, 0]; // Initial water stored in the channels
int H0[Channels] = [0, 0, 0, 0]; // Initial opening of the gates
int V0[Channels] = [0, 0, 0, 0]; // Initial inlet flow in the channels
// parameters connected with the number of irrigation or offtakes.
int Ki[Irrigations] = [1, 2, 3, 4, 1, 2, 3, 4, 3, 4, 2]; // Set of the sets of off-takes on the channels
int q[Irrigations] = [20, 15, 15, 25, 25, 13, 15, 25, 20, 13, 15]; // Quantity of water required by the off-take per time interval
int s[Irrigations] = [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]; // Desired starting time interval for the irrigation
int d[Irrigations] = [8, 8, 6, 6, 8, 6, 4, 4, 6, 6, 4]; // Desired duration for the irrigation
float alpha[Irrigations] = [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]; //priority time adequacy coefficient priority
float eps[Irrigations] ;
float beta[Irrigations] ;
float psi[Channels][Channels];
float capital_psi=0.2;

// Decision variables

dvar boolean G[Channels][TimeIntervals]; // Gate operation for channel i at time interval n on and off
dvar boolean E[Channels][Operations][TimeIntervals]; // Operation indicator for channel i at time interval n and operation m
dvar boolean F[Channels][Channels][Operations][TimeIntervals]; // Transition indicator for channels i and j at time interval n and operation m
dvar boolean S[Irrigations][TimeIntervals]; // Start of irrigation k at time interval n 
dvar float+ D[Irrigations][TimeIntervals]; // Active irrigation k at time interval n
dvar float+ V[Channels][TimeIntervals]; // Inlet volume for channel i at time interval n
dvar float+ H[Channels][TimeIntervals]; // Opening of the gate for channel i at time interval n
dvar float+ R[Channels][TimeIntervals]; // Water stored in channel i at time interval n

// Objective function
minimize 
    
  
sum(k in Irrigations) alpha[k] * (s[k]- sum(n in TimeIntervals) S[k][n]) /// maxl(S[k][1] - 1, N - s[k] - eps[k] * d[k]) 
// Volume adequacy compone
//+sum(k in Irrigations) beta[k] * q[k] * (d[k] - sum(n in TimeIntervals) D[k][n]) / ((1 - eps[k]) * q[k] * d[k]) +
 
// Water losses and extra working load component
//+sum(i in Channels, n in TimeIntervals) R[i][n] / sum(n in TimeIntervals) 
 
// Inefficiency in total workload of the gatekeeper component
+sum(i in Channels, j in Channels, n in TimeIntervals, m in Operations) psi[i][j] * F[i][j][m][n] / capital_psi
;
subject to {
  
  
  // Constraints for the max operation
   
    
  ///WATER BALANCE CONSTRAINTS
  
  //1.Aeq1 (1-\gamma_i)^{\tau_i}V_i^{n-\tau_i} +(1-\gamma_i)R_i^{n-1}- \sum_{k\in K_i} q_kD_k^n-\sum_{j\in I_i}V_j^n-R_i^n=0 
  //Aeq1: Mass balance constraint
  //Ensures that the mass balance is maintained for the water in the channels. 
  // Add the water balance constraint
forall(i in Channels, n in TimeIntervals)
  {
    //(1 - gamma[i])^tau[i] * V[i][n - tau[i]] + (1 - gamma[i]) * R[i][n - 1] -
    sum(k in Irrigations:k == Ki[i])  q[k] * D[k][n]  
    //-sum(j in Channels:j == Ii[i] )V[j][n] 
    ==0;
}  ;
}

Instead of max you could use maxl. Plus if you want to divide by a decision variable , you d better tryCPOptimizer within cplex